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    <title>Document</title>
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    <script>
        
        const num1 = NaN; // not a number 
        const num2 = 10 -5 - "abc";
        // NaN 表示计算无意义，计算单元中出现了非数字；
        console.log(num1);
        console.log(num2);
    </script>
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